Simple Door Intercom

This intercom circuit suitable for accompanying door bell. This circuit uses the speakers both as microphones and as loudspeaker. This intercom is made simple by using only a single amplifier IC LM390 to serve two-way communication. This circuit is based on master controlled half-duplex operation. With LM390, a 6V battery supply would produce loud enough volume. Here is the schematic diagram of the circuit:

The switch for this intercom circuit is ideally a push button PTT switch, but any double SPDT should work. The interaction scenario between the guest (remote) and the host (master) can be like this:

The guest press the door bell

The host press the intercom switch to talk position and then say: “who’s that?”, then release the switch to return to the listen position.

The guest then answer: “This is me bla bla bla.. “

The host press the button and say: “OK, wellcome bla bla bla..”

The simplicity of this intercom circuit is achieved by using half-duplex method, so we can use only a single amplifier for two way communication. This simplicity is further improved by utilizing the loudspeaker to act both as a microphone and as speaker. [Circuit’s schematic diagram source: National Semiconductor Application Notes]

Powerline Carrier-Current Remote Control or IntercomUsing 567 IC tone decoder/detector, we can build a remote control and or intercom. This circuit can be used to control a relay, or to carry an audio signal (intercom) through power-line wiring. Here is the schematic diagram of this … Continue reading →...

Fastest Quiz Bell/AlarmThis is aquiz bell/alarm circuit. This circuit indicate “fastest finger first”. This circuit has one bulb for Quiz master and one for each contestant. The bulb will illuminate when a button is pressed. the cathode of the 9v1 zener sees … Continue reading →...

First-Response Monitor with SCR (Quiz Alarm)A pair of cross-coupled SCRs can be used to build a first-response monitor circuit, as shown in the schematic diagram below. First-response circuit is popular in quiz application where the first contestant who press the button will prevent the other … Continue reading →...

Delayed Car Door AlarmOpening car’s door is very simple activity, and off course it’s just a daily activity which require no special skill. Almost nothing can go wrong, but forgetting to close the door when we doing or thinking on many tasks is … Continue reading →...